Crude oil and petroleum fractions in transportation, refinery separation and other processing operations often contain asphaltenes. Asphaltenes are naturally occurring in crude oil, characteristically have high molecular weight, are essentially soluble in carbon disulfide, insoluble in n-pentane, and are primarily aromatic but can contain varying amounts of sulfur, oxygen and nitrogen. Unless dissolved and/or effectively dispersed, asphaltenes can accumulate and cause fouling of equipment in which they are present.
Asphaltenes are generally dark brown to black friable solids, without a definite melting point, usually intumescent with decomposition on heating to a carbonaceous residue. They are soluble in liquids having a surface tension above 25 dyn/cm, such as pyridine, carbon disulfide, carbon tetrachloride and benzene; and insoluble in nonpolar liquids having a lower surface tension, such as low-boiling petroleum naphtha, petroleum ether, liquified petroleum gases (e.g. methane, ethane and propane), pentane, isopentane, hexane and the like. Asphaltenes are typically isolated as an insoluble residual from petroleum, heavy oil, and bitumen by the addition of 30 to 40 volumes of a non-solvent such as, for example, n-pentane, n-hexane or n-heptane.
The structure of asphaltenes is complex, including a condensed aromatic nucleus that carries alkyl and alicyclic moieties with hetero elements scattered throughout in various locations.
Chemically, asphaltenes have a typical carbon content varying from about 79 to about 89 percent by weight, and a typical hydrogen content varying from about 6.5 to about 11 percent by weight. The atomic ratio of hydrogen to carbon in asphaltenes is almost always between about 1.0 and 1.3, much lower in contrast to oils (1.5-2.0) and petroleum resins (1.4-1.6). Typical oxygen content varies from about 0.3 to about 4.9 percent by weight, sulfur content from about 0.3 to about 10.3 percent by weight, and nitrogen content from about 0.6 to about 3.3 percent by weight. Asphaltenes generally have a molecular weight higher than oils (250 to 600) and petroleum resins (700-1000), typically from about 2000 to about 8000 as measured by vapor phase osmometry.
The means by which asphaltenes are dispersed in naturally occurring petroleums, i.e. an oil medium, has been the subject of much speculation. By themsleves asphaltenes per se are insoluble in the oil fraction of petroleum, but it is reported that the dispersion of asphaltenes is mainly attributable to the naturally occurring resin fraction of the petroleum, as demonstrated by dispersing asphaltenes in oil only by adding petroleum resins. The aromaticity and the proportion of heteroatoms of the resin are important since resins of low aromaticity and/or low hetero element content are not generally as effective in dispersing the asphaltenes. A likely explanation is that the resins associate with the asphaltenes in the manner of an electron donor-acceptor and there are points of structural similarity between the asphaltenes and resins that have an adverse effect on the ability of the resins to associate with and disperse asphaltenes from a different crude oil.
Further information regarding asphaltenes, resins and oils can be found in Speight, The Chemistry and Technology of Petroleum, 2nd ed., New York: Marcel Dekker, Inc., pp. 96-97, and 404-451 which is hereby incorporated by reference herein.
Polymers comprising .alpha.-olefins and maleic anhydride are well known. Rossi U.S. Pat. No. 4,240,916 discloses an oil soluble copolymer composed of about equimolar amounts of 1-olefins and maleic anhydride useful as a lubricating oil pour point depressant wherein the 1-olefins are a mixture of C.sub.10 -C.sub.14 and C.sub.20 -C.sub.28 monomers. The pour point depressant activity is said to be enhanced by esterification of the copolymer with a C.sub.1 -C.sub.4 alcohol.
Rossi U.S. Pat. No. 4,151,069 discloses olefin-dicarboxylic anhydride copolymers and their ester derivatives having C.sub.18 -C.sub.50 linear alkyl side chains. The polymers and derivatives are said to be useful in amounts of up to 5 weight percent as filtration aids in low-temperature solvent dewaxing of waxy lubricating oils containing 5-30 weight percent wax.
Similarly, U.S. Pat. No. 3,694,176 to Miller, discloses polymers of ethylene and ethylenically unsaturated dicarboxylic acids, anhydrides or esters as wax crystal modifiers, pour point depressants and dewaxing aids in petroleum oil.
Rossi U.S. patent application, Ser. No. 515,562, filed Oct. 17, 1974, abandoned, discloses that partial alkyl ester-partial amide derivatives of low molecular weight maleic anhydride/1-olefin copolymers are useful in mineral oil lubricants as pour point depressants, viscosity index improvers and sludge inhibitors.
Japanese Kokai 62-018,494 discloses low temperature flow improvers for fuel oils which are copolymers of C.sub.20 -C.sub.28 .alpha.-olefins and maleic anhydride.
U.S. Pat. No. 3,560,456 to Hazan et al. discloses a process for making a copolymer of maleic anhydride and an aliphatic olefin having from 16-18 carbon atoms in the presence of a free radical catalyst and a solvent. The copolymer is precipitated from solution using n-propanol or isopropanol.
U.S. Pat. No. 3,231,458 to de Vries discloses a high molecular weight copolymer of .alpha.-olefins of from about 2 to about 20 carbon atoms and diolefins of from about 5 to about 20 carbon atoms reacted with maleic anhydride to form a succinic anhydride-substituted adduct said to have rust inhibiting, dispersing and thickening characteristics in liquid hydrocarbon compositions, such as fuels and lubricants.
U.S. Pat. No. 4,919,683 to Nalesnik, et al. discloses a stabilizer for a middle distillate fuel-oil which is an aromatic polyamine succinimide derivative of an ethylene/C.sub.3 -C.sub.18 .alpha.-olefin copolymer grafted with maleic anhydride.
U.S. Pat. No. 4,866,135 to Gutierrez et al. discloses a reaction product of a C.sub.5 -C.sub.9 lactone adduct of a maleic anhydride grafted ethylene/C.sub.3 -C.sub.28 .alpha.-olefin polymer with an N-containing heterocyclic aminoalkyl derivative. The polymeric lactone derivatives are said to be useful as dispersant additives for fuel and lubricating oils.
U.S. Pat. No. 4,548,725 to Bridger discloses a lubricant additive said to reduce low temperature microcrystalline wax formation in hydro-dewaxed stock made by reacting an alcohol with a maleic anhydrideolefin copolymer.
U.S. Pat. No. 5,021,498 to Stephenson et al. discloses the preparation of certain alkyl phenol/formaldehyde and lipophilic-hydrophilic polymers and their use as asphaltene dispersants-inhibitors.